Power steering systems are well known and in widespread use in cars, trucks, boats and other vehicles. One hydraulic spool valve and cylinder set used in power steering systems for marine applications is commercially available from Eaton Technologies of Eaton Rapids, Mich. This Eaton valve and cylinder set is configured for use in a boat having a linkage cable that extends between the steering wheel and tiller. A rigid rod on the end of the cable is pivotally connected to both the tiller and the valve spool. The cylinder rod is also connected to the tiller. Pressurized hydraulic fluid is provided to the spool valve by a pump that is driven by the boat engine. When the steering wheel is rotated in such a manner as to turn the boat in one direction, the cable mechanically moves the tiller in a first turn direction and simultaneously forces the valve spool to a first actuated position. In response, the valve causes the cylinder to move the tiller in the first turn direction, thereby providing hydraulic steering forces in addition to the mechanical forces provided by the cable. Similarly, when the steering wheel is rotated to turn the boat in a second and opposite direction, the cable mechanically moves the tiller in a second turn direction and simultaneously forces the valve spool to a second actuated position. The valve then causes the cylinder to move the tiller in the second turn direction to provide hydraulic steering forces in addition to the forces provided by the cable.
U.S. Pat. No. 5,028,851(Wilder) discloses a vehicle steering system that utilizes a spool valve to direct hydraulic fluid to a double-acting hydraulic ram. An electric switch on the spool valve controls a pump motor in the response to steering input torque from the steering system so that working fluid is supplied as demanded for the intended maneuver. During periods in which the vehicle may be in use without a steering maneuver being affected, the pump motor remains de-energized thereby alleviating wastage of energy and power for the vehicle. Wilder also discloses the use of a relay with a time delay in the event that the electric switch is rapidly activating and deactivating the motor.
Spool valves have a tendency to leak hydraulic fluid, placing additional strain on the pump. For some applications, such as outboard motors on boats, an unacceptable drain on the power systems can be required to compensate for leakage within the spool valve. The tight machining tolerances necessary to produce a spool valve with minimal leakage results in a system that can be cost prohibitive.